Microbial invasion of the host is followed by a series of events designed to contain and resolve the ensuing infection. First line defense against these pathogens involves components of the innate immune system. In the lung, alveolar macrophages are a major cellular component responsible for clearance of inhaled pathogens. In the absence of serum-derived opsonins in the non-inflamed lung, clearance is dependent on macrophage surface receptors and lung-specific opsonins that participate in pathogen internalization. Recent evidence has demonstrated that soluble lung-specific lectins such as surfactant associated protein A (SP-A) participate in the clearance of a number of pathogens. The focus of our recent work has been the study of the role of surfactant protein A (SP-A), one of four known surfactant-associated proteins in the lung, in mycobacterial uptake and clearance. We have shown in preliminary studies that Mycobacterium bovis BCG and Mycobacterium avium internalization by macrophages is enhanced by opsonization with SP-A. However, while SP-A increases killing of ingested BCG through a nitric oxide-dependent pathway, the presence of SP-A during M. avium internalization has no effect on M. avium survival. The goal of this project is to examine the effect of SP-A on the intramacrophage survival of mycobacteria. We are proposing to examine several steps in the mycobacterial-macrophage invasion process where SP-A might interfere with the survival strategies of the pathogen. Our hypotheses are 1) that SP-A promotes intra-macrophage killing of M. tuberculosis and BCG by altering a specific step in the phagocytic process; and 2) that SP-A enhances M. avium uptake but has no effect on intracellular survival. We will focus our studies on three specific aims: to determine if SP-A alters the intracellular trafficking of internalized mycobacteria, to characterize the intramacrophage signaling pathways activated by SP-A-mycobacterial complexes that result in production of TNF and nitric oxide, and to examine the role of SP-A in the in vivo clearance of mycobacteria. It is anticipated that results from these studies will contribute to an understanding of how mycobacteria escape normal destruction by host macrophages, and how the lung protein SP-A might overcome this escape leading to enhanced mycobacterial killing.